1. Field of the Invention
This invention relates to cameras which can function in either an ordinary or panoramic mode and, in particular, to a finder optical system for a camera which permits ordinary and panoramic photographs to be taken, independently of a photographic optical system.
2. Description of the Related Art
Recently, cameras for 35 mm film have been able to trim the upper and lower portions of the image photographed on the film by placing a light-blocking plate adjacent to the upper and lower portions of a film surface within the camera. In this way, the image on a photographic film or print can be made long sideways, and thus panoramic photography that enhances such a wide-screen sensitivity has been popularized.
Real image mode finders used in cameras which can thus bring about the states of ordinary and panoramic photographies for 35 mm full size are available in two types in connection with field frames. One is such that the frames indicative of a field range for ordinary photography and a field range for panoramic photography narrowed vertically are superposed and marked on the same field frame, which is disposed at the intermediate imaging plane of the finder optical system. In ordinary photography, a photographer recognizes the photographic range with the frame for ordinary photography, while in the panoramic photography, he does it with the frame for panoramic photography. In either state of photography, however, an image produced within the field range of the finder remains unchanged. As such, the photographer, after making sure which state of photography is desired, must determine which frame indicates the corresponding photographic range. The other type of real image mode finder, by contrast, is such that a single field frame whose shape varies according to the state of photography is disposed at the intermediate imaging plane. In ordinary photography, the field frame assumes the shape indicative of the photographic range of the state of ordinary photography, while in the panoramic photography, it changes to the shape indicative of the photographic range of the state of panoramic photography in accordance with the switching operation of the camera. Additionally, in panoramic photography, the horizontal length of the field frame is set to be longer than that in ordinary photography so that the exit angle of an eyepiece increases. The photographer can thus make sure, through the finder, that a panoramic picture will actually be taken and printed out. In this case, the focal length of the objective lens of the finder is increased in proportion to the increment of the horizontal length of the field frame, and the finder magnification is raised. This reason is as follows: an actual photographic range in panoramic photography is such that the vertical length of the field frame is reduced to a half of that in ordinary photography, while the horizontal length remains unchanged. Thus, if the focal length of the objective lens of the finder in the panoramic photography is set to be identical with that in the ordinary photography, the photographer will see the field frame of a wider photographic range than the actual one through the finder.
In the case of the former type of real image mode finder, however, both field frames will be viewed in both ordinary and panoramic photography. Hence, the photographer, when viewing through the finder, cannot determine which state of photography of the camera is in. This is very troublesome. In addition, although in the state of panoramic photography provision is made to give the corresponding indication within the finder, the photographer will be likewise liable to incorrectly identify the photographic range. Since the visual field in the state of panoramic photography is identical with that in the ordinary photography where the vertical length is merely reduced, the photographic range may be recognized, but the state of an actually printed picture is difficult to imagine. This causes the drawback that a case may arise in which the photographer's intention cannot be sufficiently reflected in the picture. In this sense, it is therefore said that the provision of a wide angle of view for the field frame is of importance to proper panoramic photography.
In the latter type of red image mode finder, in contrast, the field frame is designed so that its dimension and shape change with the state of ordinary or panoramic photography. Consequently, the photographer, when seeing through the finder, can know at a glance as to which state of the camera is brought about, and the problems described in the former type of finder are not caused. Further, in the panoramic photography, since the horizontal length of the field frame is increased so that the field frame is intended for the wide angle of view, the state of the printed picture corresponding to the wide angle of view of the objective lens can easily be imagined through the finder. In the panoramic photography, however, as mentioned above, there is the necessity for increasing the focal length of the objective lens of the finder compared with that in the ordinary photography, and thus the following difficulties are encountered. First, where the camera used is a single focusing camera, it becomes impossible to construct the objective lens of the finder with a single lens. For this reason, it is required that a concave lens is inserted in, or removed from, the objective lens to change the focal length. This inevitably increases the overall length of the finder. Second, where a zoom-lens camera is employed, the finder needs a zooming region satisfying both states of ordinary and panoramic photographies. Specifically, for example, if the horizontal length of the field frame in panoramic photography is set 1.2 times as long as that in ordinary photography, the variable magnification ratio of the finder will be required to be 1.2 times as high as that of the camera in ordinary photography only. Recently, however, in a compact zoom camera, the thickness has been minimized. In view of this development, if the finder is intended for reduction of the overall length, an increment of 1.2 times in the variable magnification ratio in addition to a zoom ratio will cause an increase in aberration or distortion and will render the finder's performance very disadvantageous. Thus, when the minimization of thickness of the camera is taken into account, the construction of the latter type of finder has the problem that the overall length of the finder is difficult to reduce and the amount of aberration or distortion increases.